A neutralized ion beam is a stream of ions with directed velocity, accompanied by an equal charge density of electrons that produces a type of net charge-neutral streaming plasma. Such neutral plasma streams allow much higher ion current density to be transported than can be transported with space-charge dominated bare ion beams.
There are many different methods used to create neutralized ion beams. There are continuous generation methods, creating steady streams of ions, and pulsed methods, creating small bursts of ions.
These sources have two basic mechanisms. The first is the creation of the ions from a non-ionic source. The second is the acceleration of the ions in a directed beam. The methods for the creation of ions can be from electric field or thermal effects. Either method results in the separation of one or more electrons from atomic or molecular elements, thus creating an ionized state. The acceleration of the ions can be caused by application of electrostatic or time dependant magnetic fields.
One class of ion sources uses a voltage potential applied between two electrodes to generate an electric field. Free electrons are accelerated by the electric field and collide with gas molecules resulting in a partially ionized gas. Often these sources use thermal effects, such as a hot filament, to increase the number of free electrons available for the ionization process.
The electric field across the partially ionized gas may add a directed velocity component to the thermal velocity of the ions, while a magnetic field may be used to focus the ion flow into a beam. Some of these sources are able to run continuously at low currents, others operate pulsed at higher currents. Sources with some of these basic characteristics are described in U.S. Pat. Nos. 6,734,434, 6,724,160, 6,717,155, and 6,696,793 respectively. For many high current applications, sources which use electrodes may not be suitable, because electrodes have a limited lifetime, may require cooling, and introduce impurities into the plasma. On the other hand, a hot filament requires a separate power supply and also has a limited lifetime.
Another type of source uses a high frequency RF transmitter with the antenna immersed in the neutral gas. The electric field component of the transmitted electromagnetic wave is used to break down the gas into plasma. This type of ion source is disclosed in U.S. Pat. No. 6,664,548. Typically, the percentage of ionization increases gradually over many cycles before the desired level is obtained. This type of source is most suitable for steady state applications.
Another type of source is inductively driven using one or more coils located near the neutral gas. The coils are driven by one or more half cycles of oscillating current. The induced electric field produced by the changing magnetic field is used to break down the gas while the magnetic field can be used to accelerate the resulting plasma. If a localized pulsed gas source is used, the ions can be accelerated away from the neutral gas, resulting in filly ionized plasma moving with a well defined leading edge. An example of a pulsed ion beam source is described in U.S. Pat. No. 5,525,805, with additional material covered in U.S. Pat. Nos. 5,656,819 and 5,532,495. Inductively driven sources eliminate the need for electrodes. For rapid ionization of gasses at low pressure with purely inductively driven sources, high rates of change in the driving current are required to produce sufficient inductive electric field for rapid, complete ionization.
All of the above examples use gas as the supply of the molecules to be broken down into ions. Other possibilities are sublimed molecules from heated solids and liquids. In all cases, the supply is generated by a pulsed or steady flow method. The supply can be near or in the breakdown region or maintains a constant background pressure of the neutral molecules in the breakdown region.
One method for generating a pulsed gas supply is to use a fast gas valve. One example, which describes a fast gas valve operated by pulsing electromagnetic coils to move a magnetic metal disk, is described in U.S. Pat. No. 4,583,710. Another valve, a fast gas valve which uses induced magnetic fields to move a non-magnetic metal disk, is described in U.S. Pat. No. 5,525,805. This type of fast valve opens and closes faster than the electromagnetic valves, but some of them have problems which reduce the quality and lifetime of the valve. In those valves, a conical metal disk was held against a seal providing an annular puff of gas. However, due to material property differences, clamping forces, adhesion forces, and other factors, one could not be assured of a uniform gas density around the annular puff. Also, deflection of the metal disk results in metal fatigue, which can limit the lifetime of the disk.
A neutral beam intensity controller is described in U.S. Pat. No. 4,596,687. Neutral beams are beams of neutral atoms, not the net-neutral ionized plasma of equal populations of free electrons and ions described in this invention, which is described in detail infra. Neutral beams are produced by charge-exchange of an un-neutralized ion beam passing through a neutral gas cell. U.S. Pat. No. 4,596,687 describes a method of controlling the current in a neutral beam by magnetic deflection of the ion beam as it enters the neutralizing gas cell. An amplitude-modulated, rotating magnetic field is applied to deflect the ions in a controlled manner to achieve the desired intensity control of the neutral beam along the beam axis at constant beam energy. The magnetic field deflects the orbits of the individual ions in the gas neutralizer before they charge-exchange to become neutral atoms, so that the peak intensity of the neutral beam is deflected away from an exit aperture downstream of the gas cell. As the magnetic deflection is increased, more of the neutrals miss the exit aperture, and the neutral beam intensity passing out through the aperture is reduced.
Therefore, it is desirous to have an improved plasma ion source in which the prior art systems' shortcomings are overcome.